The standard of the wireless local area network (WLAN) technology has been developed as the institute of electrical and electronic engineers (IEEE) 802.11 standard. The IEEE 802.11a and b use an unlicensed band at 2.4 GHz or 5 GHz, and the IEEE 802.11b provides a transmission rate of 11 Mbps while the IEEE 802.11a provides a transmission rate of 54 Mbps. The IEEE 802.11g provides a transmission rate of 54 Mbps by using orthogonal frequency division multiplexing (OFDM) at 2.4 GHz. The IEEE 802.11n provides a transmission rate of 300 Mbps for four spatial streams by using multiple input multiple output-OFDM (MIMO-OFDM). The IEEE 802.11n supports a channel bandwidth of 40 MHz, and in this case, provides a transmission rate of 600 Mbps.
The cognitive radio (CR) technology has been introduced to enhance frequency usage efficiency by using a frequency band which is not used in a specific zone at a specific time although allocated to a specific communication/broadcast service (or provider). A network that uses this CR technology will be referred to as a CR network. The CR network should not affect communication of a primary user (PU) to which a corresponding band is allocated. Accordingly, the CR network should perform in-band sensing periodically to determine whether the PU exists in a channel (in-band) currently in service. Also, if a PU signal is sensed from the channel currently in service through in-band sensing, the CR network should shift to another empty channel quickly if possible so as not to affect communication of the corresponding PU.
Currently, the IEEE 802.11af standard for defining an operation of an unlicensed device at a TV white space (TVWS) band has been developed.
The TVWS includes an ultra high frequency (UHF) band and a very high frequency (VHF) band as frequencies allocated to a broadcast TV, and means a frequency band where use of an unlicensed device is allowed on the condition that communication of a licensed device operating at the corresponding frequency band is not interrupted. A spectrum which is not used by the licensed device will be referred to as white space, and may be used by the unlicensed device. Examples of the licensed device may include devices of a system used by a licensed mode at a corresponding band, such as TV and wireless mike. The licensed device may be referred to as an incumbent user or a primary user. The unlicensed device is operated on the condition that it protects the licensed device.
The operations of all the unlicensed devices except for several special devices are allowed at 512˜608 MHz and 614˜698 MHz. However, communication between fixed devices is only allowed at 54˜60 MHz, 76˜88 MHz, 174˜216 MHz and 470˜512 MHz. The fixed device means a device that performs signal transmission at a given place only. An IEEE 802.11 TVWS user equipment means an unlicensed device operated using IEEE 802.11 media access control (MAC) layer and a physical layer (PHY) at a TVWS spectrum.
An unlicensed device which desires to use TVWS should provide a protection function for a licensed device. Accordingly, the unlicensed device should necessarily check whether the licensed device occupies a corresponding band, before starting to transmit a signal at the TVWS.
To this end, the unlicensed device may perform spectrum sensing to check whether the corresponding band is used by the licensed device. Examples of a spectrum sensing mechanism include an energy detection mechanism and a feature detection mechanism. The unlicensed device may determine that the licensed device is using a specific channel, if signal strength received from the specific channel is more than a given value, or if a DTV preamble is detected. And, if it is determined that the licensed device is being used at a channel directly adjoining a channel currently in service, the unlicensed device should lower a transmission power.
In this respect, the CR system should be designed such that in-band sensing may be performed efficiently.